Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
Cauvery Basin, ONGC, Chennai.
E.Mail: [email protected]
10th Biennial International Conference & Exposition
P 288
Log Responses of Basement Rocks in Mattur-Pundi Areas,
Tanjore Sub Basin, Cauvery Basin, India.
M.Giridhar*, N.C.Das and B.K.Choudhary
Summary
Tanjore sub basin situated on the western margin of the Cauvery Basin was evolved during Late Jurassic as a result of rifting
of Gondwanaland. Basement rocks are mostly of gneissic and at places granitic in composition in Tanjore sub basin where
hydrocarbon has been produced. Extensive study on the effects of compositional variations on log responses of igneous and
metamorphic rocks was done by Peching et al., (2005). Similar analysis has been attempted in Mattur-Pundi areas to
understand log responses of basement rocks. Comparative study between the logs of Mattur-Pundi areas shows that in Mattur
area resistivities are higher with low gamma ray values compared to Pundi area where the gamma ray values are higher with
low resistivity values although both the fields produced hydrocarbon from Basement. Spectral gamma ray log shows high
Potassium and Thorium contents in Pundi area compared to Mattur area. The variation in log character of these two areas
may be due to compositional variation of rock types being mostly gneissic in Mattur area and granitic in Pundi area. As the
reservoir character of granitic rocks are generally better than gneissic rocks the understanding of log characters in relation
to the type of basement would help in designing exploration/production testing strategy.
Keywords: Gneisses, granites, Mattur, Pundi, resistivity and radioactive logs
Introduction
Electrical and radioactive logs are recorded routinely in
all the wells drilled during hydrocarbon exploration.
Evaluation of logs is done primarily for sedimentary rocks
to know the lithology, hydrocarbon saturation and
porosity. A lot of literature is available on the formation
evaluation of sedimentary rocks, whereas for the
basement rocks the evaluation of logs to understand the
type of basement based on the log response is scanty.
Peching, et.al. (2005) have compiled an extensive data of
log measurements of the boreholes drilled into crystalline
basement of European and North American provinces.
Study was focussed to understand compositional
variations of igneous and metamorphic rocks. From the
logs potassium and neutron porosity are particularly
helpful in distinguishing different types of gneisses and
igneous rocks. The proportions of amphibole/pyroxene,
mica-feldspar has influence on log responses. The
gamma-ray values cover a large value range (40-550)
within the plutonic rocks and gneisses. The average
gamma ray values of plutonic rocks are about 250API for
granites/aplites and about 350API for the
Fig.1: Tectonic map of Cauvery Basin showing different sub
basins and ridges
2
syenites/monzonites and are significantly higher than the
gamma ray mean values of all gneisses(70-200API). Mean
electical resistivities for plutonic rocks (2.1-3.9 Ohm.m.)
are lower than for ortho gneisses(3.2-4.5 Ohm.m.) as
reported in their study.
Tako Koning(2012) after extensive study on the basement
rocks of Asian countries has reported that quartzites and
granites form optimum reservoirs. Weathered “rotten”
granites can also be excellent reservoirs. Rocks such as
schists and gneisses are less attractive and tend to smear
and not fracture when subjected to tectonic stresses. The
high mafic content of schists also negates the creation of
secondary porosity by weathering. In this paper an attempt
was made to understand lithological variation between
basement rocks based on the log responses in Tanjore sub
basin.
Basement configuration of Tanjore sub basin
Cauvery basi was evolved as a result of rift-drift
phenomenon of Indian plate from Gondwana land during
Late Jurassic-Early Cretaceous with taphrogenic
fragmentation of Archean Basement which resulted in the
development of faults and has divided the basin in to horsts
and grabens/half grabens. In Tanjore sub-basin which lies
on the western part of the Cauvery basin(Fig.1), the
western margin fault has served as foot wall and hanging
wall has moved down giving the half graben structure.
There are four prominent lows at basement level, one NE
of Vadakkukkottai-1, one around orathanadu-1, two lows
north of Sattanur-1 and the basement is shallower towards
south, east and northern parts of the sub-basin(Figs.2,2a).
Generalized stratigraphy of Tanjore sub basin showing
hydrocarbon occurrences is shown in Table 1.
Fig 2: 3D-Perspective of Basement Horizon – Thanjavur Acreage
3
Basement faults are aligned in NE-SW direction in the
main basinal low whereas they are aligned in the direction
of ENE-WSW in the northern part in Mattur-Pundi areas.
Southernmost part of the basin is also dissected by number
of smaller faults which are aligned in NW-SE direction.
The basement in Tanjore sub-basin is mostly granitic
gneiss or granite with schistose rocks. The hanging wall is
dissected into a series of small half grabens by east-hading
antithetic faults trending NE-SW towards eastern side and
parallel to the Pattukottai-Mannargudi ridge. Towards
eastern side the basement is steep close to Pattukkottai
ridge and becomes gentle towards central parts and deep
towards western side giving half graben structure.
The depth of basement is shallowest towards north around
Mattur-Pundi areas(1000-1200m) and shallowest towards
south around Krishnapuram area ( <2300m) whereas it is
deepest towards north of Sattanur (5500-6000m),
Vadakkukkottai low (>5000m) and Orathandu
low(>5000m) and the depth increases towards west(Fig.3)
FIG:2a 3D-Perspective Of Basement Horizon – Mattur-Pundi Area
4
Fig.3: Structure contour map of Tanjore sub basin showing fault pattern at basement level
5
Compositional variation of logs in Basement rocks
Around 30 wells have penetrated Basement rocks in
Tanjore sub basin. Only those wells which have penetrated
more than 80m have been considered for the study
representing true basement. Basement rocks are mostly
Biotite-Hornblende gneisses in composition with granites
at places (Vadatheru area).Even phyllites have been
reported(Mattur-6). Since in Mattur-Pundi wells a
considerable thickness has been drilled a comparison
between these two areas has been taken for the
study(Table.2) The gamma ray values for Mattur wells are
low and vary between 20-50 API compared to Pundi wells
( 60-125 API). Resistivity values for Mattur wells vary
between 80-200 Ohm.m. and for Pundi wells between 20-
50 Ohm.m. having considerable differences between these
areas(Figs.5,6,).The spectral gamma ray also corroborates
with this observation having low potassium(K) and
Thorium(Th) contents in Mattur-K well compared with
PU-D well which has high Potassium and Thorium
contents(Figs.7,8) The densities vary between 2.7-2.8 g/cc
for Mattur area whereas for Pundi area it is between 2.6-
2.7. The neutron porosities are better for Pundi area than
Mattur area. Scatter diagram between gamma ray and
resistivity values show difference between two
areas(Fig.4).Incidentally Pundi-B has produced
12,000tons of oil in comparision to Mattur-B which
produced only 2000tons of oil supporting that in Pundi area
the basement is granitic in nature with better fracture
development. Alangudi-A well which is close to Mattur
area where 400m basement has been drilled is
characterised by garnetiferous biotite gneiss the gamma
ray values range between 30-40API and has an average
resistivity value of 200 Ohm.m supporting above
observation. Similar studies can be attempted for the
entire basin where the basement rock has been drilled for
considerable depths.
6
Fig.4: Cluster diagram showing high gamma ray and low resistivity values in Pundi wells in
comparison to Mattur wells
7
Fig.5 Composite log showing high GR values, low Rt values, high Potassium, Thorium contents in PU-D well
8
Fig.6 Composite log showing low GR values high Rt values, low Potassium, Thorium contents in MT-K well
9
Fig.7: Depth vs Potassium plots showing high potassium content in Pundi well
Fig.8: Depth vs Thorium plots showing high Thorium content in Pundi well
10
Conclusions
1. Gamma ray and Resistivity values between Mattur
and Pundi areas show variation in Basement rocks.
2. Low gamma ray, high resistivity, low potassium and
Thorium contents in Mattur wells and high gamma
ray, low resistivity, high potassium and Thorium
contents in Pundi wells have been observed.
3. Based on the differences Mattur area is mostly
gneissic in composition and in Pundi area is mostly
granitic in composition.
4. As quartzites and granites serve as good reservoir
rocks compared to gneissic rocks a detailed study as
regards the nature of Basement would help in
designing exploration and production strategy as
quantities of hydrocarbon accumulations may vary
depending on the type of rock.
Acknowledgements
The authors express their thanks to Dr.B.S.Josyulu,
Executive Director-Basin Manager, Cauvery Basin for
encouragement, cooperation to carry out such studies.
Authors express their gratitude to Mr. S. Prabakaran, GM
(Geol)-Block Manager for going through the manuscript
and making suggestions. They are grateful to SPG
authorities for conducting the conference and allowing the
paper to be presented in the proceedings.
References
Peching,R, Delius,H and Bartetzko, A(2005)” Effect of
compositional variations on log responses of igneous and
metamorphic rocks. II:acid and intermediate rocks”
Petrophysical Properties of Crystalline
Rocks(eds).Geological Society, London, Special
Publications,240,279-300.
Tako Koning(2012) “Best practices for exploring and
producing oil and gas from fractured and weathered
basement: Examples from Asia” Search and Discovery
article AAPG International conference and Exhibition,
Singapore, September,16-19,2012.
Well completion reports, laboratory reports submitted by
different agencies of ONGC (Unpublished data)